1. 1 Chapter 9 Distillation

2. 2 [1. FLASH DISTILLATION( 闪蒸 )] 2.CONTINUOUS DISTILLATION WITH REFLUX 回流 ( 蒸馏 ) [Action on an ideal plate] [Combination rectification and stripping (Fig.9.4-3)] [Trap: 疏水器 ; accumulator: 集液器 ; condensate: 冷凝液 ; fractionating: 精馏 ; reboiler: 再沸器 ]

3. 3 Control surface 1 Control surface 2 Feed Condenser Accumulator Heat out Overhead product Bottoms product Reboiler Reflux Heat in [Fig.9.4-4(p.104)]

4. 4 1)Material Balances in Plate Columns (1) Overall and more volatile material balances for two- component systems (9.4-1) (9.4-2) Units of F, D, B: kmol/h, kg/h, lb/h x D, x F, x B : molar fraction, or mass fraction, dimensionless x D, x F, x B : molar fraction, or mass fraction, dimensionless

5. 5 Eliminating B from eq.(9.4-1) and (9.4-2) gives Eliminating D gives

6. 6 (2)Net flow rates Net flow rate in the upper section /rectifying sectionNet flow rate in the upper section /rectifying section Quantity D is the net flow rate of material, Dx D is the net flow rate of component A, all upward in the upper/rectifying section of the column. Regardless of changes in V and L, and x, y, the differences are constant and equal to D, and Dx D, respectively.Quantity D is the net flow rate of material, Dx D is the net flow rate of component A, all upward in the upper/rectifying section of the column. Regardless of changes in V and L, and x, y, the differences are constant and equal to D, and Dx D, respectively.

7. 7 Net flow rate in the lower/stripping sectionNet flow rate in the lower/stripping section The net flow rates are also constant but are in a downward direction. The net flow rate of total material equals B; that of component A is Bx B.The net flow rates are also constant but are in a downward direction. The net flow rate of total material equals B; that of component A is Bx B.

8. 8 (3) Operating lines Two sections two operating linesTwo sections two operating lines Operating line for rectifying sectionOperating line for rectifying section (9.4-10 ) (9.4-11 ) (9.4-12 ) Slope of operating line in rectifying section less than 1.0

9. 9 Operating line for stripping sectionOperating line for stripping section (9.4-13 ) (9.4-14 ) (9.4-15 ) Slope of operating line in stripping section greater than 1.0

10. 10 2)Number of Ideal Plates;McCabe-Thiele Method McCabe-Thiele step-by-step construction for computing number of ideal plates （逐板计算法） : Using equilibrium curve and operating lines.McCabe-Thiele step-by-step construction for computing number of ideal plates （逐板计算法） : Using equilibrium curve and operating lines. (1)Constant molal overflow 恒摩尔溢流 Conditions: (1)nearly equal molar heats of vaporization of every component, (2)Sensible heats （显热） and heat losses （热损失） omitted. the molar flow rates of vapor and liquid are nearly constant in each section of the column, and operating lines are almost straight.

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12. 12 Constant molal overflow of vapor （恒摩尔气流） Constant molal overflow of liquid （恒摩尔液流） Units of above parameters: kmol/h

13. 13 Operating lines becomes Rectifying section Stripping section

14. 14 (2)Reflux ratio: R D or R V In this text only R D will be used. (9.4-18)

15. 15 Construction of operating line of rectifying section: When y intercept （截距） :

16. 16 operating line of rectifying section

17. 17 (3)Condenser and top plate Plate1 Total condenser （a）（a）（a）（a） （b）（b）（b）（b） （c）（c）（c）（c）

18. 18 When a total condenser is used, triangle （三角形） abc in Fig.9.4-6a represents the top plate in the column.When a total condenser is used, triangle （三角形） abc in Fig.9.4-6a represents the top plate in the column. (a) （b）（b）（b）（b） Total condenser

19. 19 Question: Why is the partial condenser used?Question: Why is the partial condenser used? For a partial condenser,x C is in equilibrium with y’, or x D. The partial condenser, represented by the dotted triangle a’b ’ c ’ in Fig.9.4-6b, is equivalent to a theoretical plate. [ 分凝器相当于一块理论板 ]For a partial condenser,x C is in equilibrium with y’, or x D. The partial condenser, represented by the dotted triangle a’b ’ c ’ in Fig.9.4-6b, is equivalent to a theoretical plate. [ 分凝器相当于一块理论板 ] (b)

20. 20 (y 1, x C ) is on operating line of rectifying section. (y 1, x C ) 的交点在操作线上。(y 1, x C ) is on operating line of rectifying section. (y 1, x C ) 的交点在操作线上。 (b)(b)(b)(b)

21. 21 When a partial condenser is used, triangle a’b’c’ in Fig.9.4-6b represents the top plate in the column.When a partial condenser is used, triangle a’b’c’ in Fig.9.4-6b represents the top plate in the column.

22. 22 When condensate is liquid at its bubble point, L=L C, V=V 1.When condensate is liquid at its bubble point, L=L C, V=V 1. If the reflux is cooled below the bubble point, a portion of the vapor coming to plate 1 must condense to heat the reflux; so V 1 L C.If the reflux is cooled below the bubble point, a portion of the vapor coming to plate 1 must condense to heat the reflux; so V 1 L C. Plate1 Total condenser （a）（a）（a）（a） （b）（b）（b）（b） （c）（c）（c）（c）

23. 23 the reflux is cooled below the bubble point, so V 1 L C.the reflux is cooled below the bubble point, so V 1 L C. =specific heat of condensate =temperature of liquid on top plate =temperature of returned condensate = heat of vaporization of condensate The actual reflux ratio in the column is thenThe actual reflux ratio in the column is then

24. 24 (4)Bottom plate and reboiler For constant molar overflowFor constant molar overflow (9.4-21 ) WhenWhen Construction of operating line of stripping sectionConstruction of operating line of stripping section

25. 25 WhenWhen Construction of operating line of stripping sectionConstruction of operating line of stripping section 提馏段操作线过点（ x B ， x B ）

26. 26 The vapor leaving the reboiler （ yr ） is in equilibrium with the liquid leaving as bottom product （ x B ）. The reboiler acts as an ideal plate 再沸器相当于一块 理论板.The vapor leaving the reboiler （ yr ） is in equilibrium with the liquid leaving as bottom product （ x B ）. The reboiler acts as an ideal plate 再沸器相当于一块 理论板. ybyb 塔底集液层

27. 27 At the plate where the feed is admitted, the liquid rate or the vapor rate or both may change, depending on the thermal condition of the feed （进料热状态）.At the plate where the feed is admitted, the liquid rate or the vapor rate or both may change, depending on the thermal condition of the feed （进料热状态）. Five different feed types: (a) feed cold liquid 冷液进料 ; (b) feed saturated liquid 饱和液体 进料 ; (c) feed partially vaporized 汽液混合物 进料 ; (d) feed saturated vapor 饱和蒸汽进料 ; (e) feed superheated vapor 过热蒸汽进料.Five different feed types: (a) feed cold liquid 冷液进料 ; (b) feed saturated liquid 饱和液体 进料 ; (c) feed partially vaporized 汽液混合物 进料 ; (d) feed saturated vapor 饱和蒸汽进料 ; (e) feed superheated vapor 过热蒸汽进料. (5)Feed plate

28. 28 Thermal condition of feed 进料热状况.Thermal condition of feed 进料热状况. Material balance about feed plateMaterial balance about feed plate Enthalpy balance of feed plateEnthalpy balance of feed plate Hence Define

29. 29 q = parameter of thermal condition of feed 进料热 状况参数, or moles of liquid to stripping section of column per mole of feed 进料液化率 For cold-liquid feed Define (9.4-22)

30. 30 = specific heats of liquid and vapor, respectively = temperature of feed = bubble point （泡点） and dew point （露点） of feed, respectively = heat of vaporization For partially vapor feed, 0<q<1For partially vapor feed, 0<q<1 For cold-liquid feed

31. 31 Define For superheated vapor feed (9.4-23)

32. 32 Material balance of feed plateMaterial balance of feed plate = fraction of feed that is vaporized 进料 汽化率 (9.4-25)

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34. 34 q Values of five feed types Cold feed

35. 35 q Values of five feed types Feed at bubble point (saturated liquid 饱和液体 )

36. 36 q Values of five feed types Feed partially vapor

37. 37 q Values of five feed types Feed at dew point (saturated vapor)

38. 38 q Values of five feed types Feed superheated vapor

39. 39 Question: Bubble point feeds,

40. 40 Feed line 进料线 ----Line of intersections ( 交点 ) of the two operating lines.Feed line 进料线 ----Line of intersections ( 交点 ) of the two operating lines. (6)Feed line

41. 41 For constant molal overflow （恒摩尔流）, the operating line equations for two sections areFor constant molal overflow （恒摩尔流）, the operating line equations for two sections are Rectifying section Stripping section

42. 42 (9.4-26) (9.4-27) That is To locate the point where the operating lines intersect, let y n+1 =y m+1 =y, x n =x m =x and subtract Eq.(9.4-27) from Eq.(9.4-26) :

43. 43 Eq.(21.31)represents a straight line, called the feed line, on which all intersections of the operating lines must fall.Eq.(21.31)represents a straight line, called the feed line, on which all intersections of the operating lines must fall. The position of the line depends only on x F and q.The position of the line depends only on x F and q.

44. 44 (7) Construction of operating lines The simplest method of plotting the operating lines is toThe simplest method of plotting the operating lines is to (1)locate the feed line:

45. 45 (2) Construction of operating line of rectifying section: When y intercept: operating line of rectifying section

46. 46 (3) Draw the stripping line through point (x B,x B ) and the intersection of the rectifying line with the feed line.

47. 47 (8) [Optimum] Feed plate location （适宜的进料位置） When should the steps transfer from the rectifying line to the stripping line? The change should be made in such a manner that the maximum enrichment per plate is obtained, so that the number of plates is as small as possible. The transfer should be made immediately after a value of x is reached that is less than the x coordinate ( 坐标 ) of the intersection of the two operating lines.

48. 48 How many ideal plates are needed? Reboiler? Partial condenser? Is the number of ideal plates related with feed flow rate F? 3 Feed plate 5 Optimum feed plate location? xFxFxFxF xDxDxDxD xBxBxBxB Optimum feed plate location: plate 3

49. 49 Problem A continuous rectifying column treats a mixture consisting of 40% weight benzene and 60wt% toluene at the rate of 4 kg/s, and separates it into a product 97 % benzene on the top of column and a liquid containing 98wt% toluene at the bottom of column. The feed is liquid at boiling point. (a) Calculate the weights of distillate and waste liquor per unit time. (b) If a reflux ratio of 3.5 is employed, how many plates are required in the rectifying part of the column ? (c) What is the actual number of plates if the plate efficiency is 60%?

50. 50 Mol fraction of benzene in liquid: 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Mol fraction of benzene in vapour : 0.22 0.38 0.51 0.63 0.7 0.78 0.85 0.91 0.96

51. 51 Solution The equilibrium, data are plotted on Fig. llh. As the compositions are given as weight %, they must first be converted to mol fractions before the McCabe-Thiele method can be used. Mol fraction benzene in feed Similarly, xd=0.974 and xw=0.024

52. 52 [EXAMPLE 21.2(a)(b)][EXAMPLE 21.2(a)(b)]

53. 53 As the feed is a liquid at its boiling-point, the q-line is vertical and may be drawn at xf =0.44. (a) A mass balance over the column and on the more volatile component gives in terms of mass flow rates: 4.0 = W' + D' 4× 0-4= 0.02W' + 0.97D' from which: W' gives = bottoms flow rate = 2.4 kg/s And: D'=top product rate = 1.6 kg/s '

54. 54 (b) If R= 3.5, the intercept of the top operating line on the y-axis is given by xd /(R+1) = 0.974/4.5 = 0.216, and thus the operating lines may be drawn as shown in Fig.11h. The plates are stepped off as indicated and 10 theoretical plates are required. (b) If the efficiency is 60%, the number of actual plates = 10/0.6 = 16.7 17 actual plates

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